The subject invention relates to inertial navigation, and more particularly to aided inertial navigation systems (AINS) based Land Surveyor Systems which use an AINS as a navigational reference and which make it possible to survey forested areas where GPS signals may be missing for long intervals of time or indefinitely due to foliage and or a dense tree canopy. Land surveyor systems that are not aided with a radio positioning system such as GPS as a result of signal blockage due to a tree canopy tend to accumulate position error linearly as a function of time.
AINS technology originated in the late 1960xe2x80x2s and found application on military navigation systems. An example or the many books on the subject is the text by George Siouris, titled xe2x80x9cAerospace Avionics Systems, A Modern Synthesisxe2x80x9d, published by Academic Press, published in 1993.
Traditional methods of surveying used laser theodolites, which required access to lines of sight between positions to be surveyed. A line of sight is not always available in forested areas. A line of sight was typically obtained in the past by bulldozing corridors 3-4 meters wide through the forest along the lines or paths to be surveyed. Governments discourage this wasteful and environmentally destructive practice by imposing stumpage fees on destroyed trees and other forms of penalties for environmental damage. The advent of a precise GPS has provided an alternative method of land surveying; however, a Land Survey System based exclusively on a precise GPS receiver is limited to areas where the sky is visible. Some tropical rain forests have canopies that are so dense that precise GPS cannot be used at all.
AINS Land Surveyor
An AINS land surveyor does not require access to the sky and can be operated under a dense tree canopy. A single surveyor using an INS land surveyor can map a forest or jungle area by walking a path among the trees thereby avoiding the need to cut trees to establish a survey lane as would be required using theodolites or a precise GPS. Use of an AINS land surveyor therefore results in reduced cost, and the environmental impact of the survey is substantially reduced.
To reduce drift and obtain acceptable accuracy, the surveyor using an AINS-based land surveyor systems brings the INS to a complete rest about every 1-2 minutes for a period of 5-30 seconds. This is called a zero-velocity update (ZUPD). A Kalman filter uses each ZUPD to zero the INS velocity error and partially calibrate out inertial sensor errors. The position error drift with periodic ZUPD""s is on the order of 0.5-2 meters per kilometer, depending on the quality of the inertial sensors and on the frequency of ZUPD""s. The requirement for ZUPD""s is often an inconvenience, since it limits the surveyor""s production.
The Kalman filter in an AINS based Land Surveyor System is also coupled to receive measurement inputs from one or more external and independent sources which are processed to further refine or aid the navigation solution. Aiding information can include aiding signals from sources such as a precise GPS receiver, a Doppler Radar, or from an odometer or a precise pedometer. In the survey of an area permitting an unobstructed view of the sky, a precise GPS provides the simplest aiding signal. Several different accuracy levels are associated with GPS performance and signals. C/A GPS implies uncorrected GPS, and provides 10-20 meter 3D position accuracy. A GPS can be coupled via a radio modem link to a base GPS receiver, which has a precisely known position. The base GPS measures the errors in the signals being received and forwards corrections via the model to the precise GPS aboard the navigator. An RTCM-corrected differential GPS uses industry standard RTCM differential corrections from a dedicated base receiver or a differential corrections service such as the U.S. Coast Guard or Omnistar to obtain 0.5-1.0 meters position accuracy. Real-time kinematic (RTK) GPS uses differential GPS data from a dedicated base receiver to obtain 0.05-0.1 meters position accuracy.
The invention system allows a surveyor to carry the invention AINS-based Land Surveyor System with Reprocessing along a predetermined path or line and to locate and record the position of stakes that the surveyor positions in accordance with a pre-planned grid of locations. The stakes identify the locations of sensors and explosive charges that are used in a seismic survey. The predetermined path begins and ends at position fix (PF) locations that are precisely known. The invention AINS-based Land Surveyor System with Reprocessing then uses the known PF locations at each end of the path and a smoother algorithm to reprocess the stake locations.
Seismic Exploration
Seismic exploration has as its object, the production of a multi-dimensional map of the geological structure over an area below the ground for the purpose of identifying valuable oil, gas and mineral deposits. A seismic survey uses acoustic interferometry to perform the multi-dimensional subterranean mapping. A geophysicist provides a pre-plot map with grid locations of the desired positions of the noise sources and the geophones over the space to be explored. The noise sources are multiple phased dynamite explosions on a 2-dimensional grid pattern. The sound waves from the charges are reflected by the different geological strata, and received by an array of geophones on a separate 2-dimensional grid connected to recording devices.
The multi-dimensional geological map is generated by post-processing the recorded data. The typical error specification for the position of a noise source or a geophone is one meter horizontal and 0.5 meters vertical.
A backpack-borne AINS based system is made and sold by the assignee of this application, Applanix Corporation. at 85 Leek Cresent, Richmond Hill, Ontario, Canada L4B 3B3. The system is called the Position and Orientation System for Land Survey (POS/LS). In operation, a surveyor walks a survey path or trajectory carrying the POS/LS as a backpack. Such survey trajectories often pass through areas where GPS signals are not available. The POS/LS navigates though such GPS outage areas in a dead-reckoning mode with as little position drift as possible. Typically the surveyor moves from one known position to another, and xe2x80x9cties-downxe2x80x9d or xe2x80x9cfixesxe2x80x9d the POS/LS position at the known positions.
The path followed by a surveyor is typically a zigzag pattern of parallel seismic lines, each 1000-5000 meters long with the stakes positioned every 50-200 meters on the lines. A control survey is sometimes performed to verify the accuracy of the seismic survey. A control survey is formed by a number of short survey paths between known positions with traverse legs being approximately perpendicular to the principal seismic or grid lines. Intersections of the control paths with the seismic lines form the control points. The control point position accuracies should be significantly better than the seismic line position accuracies.
This invention teaches the use of an optimal smoothing algorithm and a method of reprocessing the data accumulated and stored from an AINS-Land Surveyor System with Reprocessing (AINS-LSSRP) between known position fixes or xe2x80x9ctie-downsxe2x80x9d.
A first embodiment of the AINS-LSSRP comprises an AINS that provides a sequence time-indexed present position values in response to the LSSRP being moved from a first known position value or PF at the start of a survey interval to a second known position value at the end of the survey interval. The LSSRP has a Position and Orientation (POS) Computer Subsystem (PCS) coupled to receive and store the sequence of time-indexed present position values as the surveyor moves from the first known position to the second known position. The PCS has a reprocessing computer and program means for processing the indexed present position values with a smoothing algorithm to provide indexed and adjusted present position values for at least some of the indexed present position values recorded as the LSSRP was moved from and between the first known position value at the start of the survey interval and the second known position value at the end of the survey interval.
In a more particular alternative embodiment, of the LSSRP, the AINS uses a Kalman filter responsive to at least two sources of aiding signals, and the PCS has an aiding signal selector algorithm characterized to select the most accurate aiding signal for use by the Kalman filter from all available aiding signals.
In yet another more particular embodiment of the LSSRP, the AINS uses a reprocessing computer and program means smoothing algorithm that is used is a Modified Bryson-Frazier smoother (MBFS).